Community-driven study site grants access to millions of textbook solutions. Also offers Q&A and peer tutoring. Users can find pre-existing textbook solutions or set a "bounty" and pay in "gold" that's either earned or purchased. Features

AllMyFaves: "If you're having trouble with your homework, or if you just need a little more explanation than your teacher offered in class, Slader is the virtual tutor that'll make your life much easier."

iTunes Store: 3.5 out of 5. Users say the app is very helpful but express annoyance with ads that cause their music to pause and pop up ads displayed every few questions.

Techcrunch: "Koofers is driving the reinvention of our university system."

Entrepreneur: "By logging into Koofers--which has a Facebook app and is accessible online via Facebook ID and password - a student in an introductory chemistry class of 30 can now swap and share materials with tens of thousands of students studying the same curriculum worldwide."

Education World: 5 out of 5. "StudyBlue is 100% free to use, but unlike other free online tools, the digital flashcards provided here look and work like those for which others happily pay."

EdTechTimes: "Overall the StudyBlue system is a great tool with a poor web presence. That's neither here nor there though, as one can use all the robust features without ever visiting the website at all."

Reviewopedia: 1.5 out of 5. "Overall, online reviews displayed negative feedback on the services provided by Course Hero. If you are paying for an education it is probably best just to do the work yourself or join a study group."

Geek And Nerd: 7.5 out of 10. "The quality of the resources available is very high. In this coursehero.com review, it is worth mentioning that you can readily find resources for a specific course at a particular college or university."

11. Study.comView short online videos to help prepare for a test or learn new materials geared toward students from middle school through adult professional education. Can also earn college credit with online video courses. Features

Offers open education resources, classroom core content instruction, and study help for college students. Over 5,700 videos available in 13 subject areas. Features

Multimedia approach

Exam prep

Warm-up activities

Sample quizzes

Sample problems

Videos

App

Not available

How much does HippoCampus cost?

Free for individual users

HippoCampus Reviews

Common Sense Media: 3 out of 5. "HippoCampus provides credible information on a wide range of topics, although some of the lessons are more enjoyable than others."

Good Housekeeping Institute: 3.5 out of 5. "HippoCampus.org was the highest-rated website for assisting students with multiple subject areas because it offers video, audio, and written tutorials, plenty of examples, and interactive options."

14. StudyStackCreate flashcards, study user-submitted flashcards, organize flashcards in "stacks," and participate in learning activities generated from the flashcards. Features

The Idaho Librarian: 4 out of 5. "Flashcard Machine is not flashy, but it's flexible and completely free."

19. CramberryCramberry offers a simple premise: "studying doesn't have to be painful." The site provides online flashcards in a variety of subjects, including Science, History, Languages, Accounting, and Medicine. Features

Flashcards

Make your own flashcards or study millions of user-submitted flashcards

Tracks your progress and serves the flash cards you're struggling with more often

EdShelf: 4 out of 5 for "learning curve" and "pedagogical effectiveness," 5 out of 5 for "student engagement." "… a refreshingly new take on introducing a subject and formative assessment…"

Center for Teaching Quality: "Kahoot is a free service that allows users to create and deliver online quizzes, surveys and discussions in a competitive game-based environment… it is a lot of fun to play."

App Ed Review: 7.5 out of 10. "Kahoot is a tool designed for users to create digital, game-like assessments."

Ed Tech Review: "Learners of all age can benefit from Kahoot. Today, many students possess smart devices, and by using Kahoot on their own devices, students can actually make group learning fun."

This nonprofit organization championed by Bill Gates offers a free learning system covering a wide variety of subjects: Math, Science and Engineering, Computing, Arts and Humanities, Economics and Finance, and test prep. Features

PCMag: 4 out of 5. "Generally helpful tutorials paired with large database of machine-graded assessments. Video markup can be onerous. Assessment is impressive in scale, but ill-suited to some subjects."

Consumer Affairs: 4 out of 5. Reviewers cited a large course selection and ease-of-learning as pros, though some reviewers noted a lack of repetition and a tendency to become bored as potential cons.

Site Jabber: 4 out of 5. Most reviews were positive and stated Khan Academy makes learning easy. The few negative reviews complained that the lessons lose their fun factor after a while.

Success at School: "One thing that Khan Academy does really well is explain complicated theories in simple terms."

A literature study guide search engine that allows users to search learning resources for more than 44,000 books from sources like SparkNotes, Pink Monkey, Cliff's Notes, Shmoop, and more from one place. Features

A points-based Q&A system: ask questions and spend points when they're answered; earn points for answering questions. Move up the ranks with gamification features and earn more points for being active in the community. Features

An online memorization tool that lets you create study pages in multiple choice, matching, or flashcard modes. You can also collaborate with others. The site is not continually updated, but it has good study resources.Features

Use wizards to create pages

Memorize with different modes

Share pages

Upload diagrams, images, links, etc.

Share with others

Apps

Not currently

How much does memorize cost?

Free

Memorize Reviews

G2Crowd: 4 out of 5. "Memorize.com pages have interactive learning modes. They make facts clear and lucid. Create a page in seconds! Share or assign it."

instaGrok lets you esearch study topics and create an interactive concept map that can be customized and shared. It's like a cross between a mind map and a study guide; a visual tool that helps you link key concepts and learn more efficiently.Features

Common Sense Media: 3 out of 5. "the site does a good job of eliminating inappropriate links or sources from a search. Another bonus is the fact that you can run all your chosen sources through EasyBib, a site that verifies research sources."

Although a bit dated, this site has tons of valuable information on how to learn and study. Rather than helping you study for specific courses and quizzes, the site teaches you how to study and introduces various study methods so you can learn more efficiently with improved retention. That said, there are some subject-specific tutorials available.Features